1. Field of the Invention
The present invention relates to a method and apparatus for distance measurement.
2. Description of the Related Art
Distance measurement techniques exist in which acoustic signals in the ultra-sonic range are transmitted in the form of short pulses by a narrow band transmitter and are detected by a sensor. In such devices, the interval between pulses must be longer than the time taken for the ultra-sonic signal to travel between the transmitter and the detector in order to avoid ambiguity in pulses detected at the sensor. This in turn limits the distance over which the device can be used or the speed of response.
Preferred embodiments of the present invention seek to overcome the above drawback of the prior art.
According to an aspect of the present invention, there is provided a distance measuring method comprising the steps of transmitting between at least one pair of first and second locations a respective ultrasonic signal, transmitting between the or each pair of first and second locations a respective radio frequency signal, and determining the distance between the first and second locations of the or each said pair of first and second locations from the difference in propagation time between said ultrasonic and radio frequency signals.
The method may further comprise the step of rotating the or each said ultrasonic signal. This may be achieved by mechanical means such as a motor or electronic means such as a phased array of transmitters.
This provides the advantage of enabling a position vector between a transmitter and a detector to be determined.
The or each said ultrasonic and radio frequency signal may be emitted from a respective first location and detected at a respective second location. Alternatively, the ultrasonic and radio frequency signals may be emitted from different locations, and suitable means used to exchange information between the locations.
Preferably, the or each said ultrasonic signal includes a respective first modulation signal having a repetition period longer than the propagation time of said ultrasonic signal between the respective first and second locations.
By providing a first modulation signal having a repetition period long compared with the propagation time of the respective ultrasonic signal, the detected wave form of the first modulation signal provides a unique indication of the time delay between transmission and detection of the said ultrasonic signal, thus avoiding by relatively simple means the problems of ambiguity associated with the prior art.
In a preferred embodiment, the or each said radio frequency signal includes a respective second modulation signal associated with said first modulation signal.
By providing one or more second modulation signals related to the respective first modulation signals, this has the advantage of enabling synchronisation by relatively simple means between the respective transmitters and detectors.
Preferably, the step of detecting the or each said radio frequency signal further comprises deriving the respective second modulation signal, generating at the respective second location a plurality of reference signals corresponding to the respective first modulation signals delayed by substantially constant time intervals relative to each other, and comparing a signal derived from the or each said detected ultrasonic signal with each of the plurality of reference signals.
This provides the advantage that by having a sufficient number of reference signals at the or each second location, detection of substantial correlation between the signals derived from the respective ultrasonic signals and the reference signals indicates the approximate time delay between receipt of respective ultrasonic and radio frequency signals. This in turn provides the advantage that sophisticated memory devices are not required in order to determine the difference in propagation time between respective ultrasonic and radio frequency signals.
In a preferred embodiment, the step of generating said reference signals comprises generating respective first modulation signals at the or each second location and inputting each said first modulation signal to a respective shift register having a plurality of outputs.
The method may further comprise the step of determining a degree of correlation between the or each signal derived from the detected ultrasonic signal and each reference signal.
This provides the advantage that when the difference in transmission time between respective ultrasonic and radio frequency signals lies between two of the intervals corresponding to the reference signals, this is readily indicated by correlation between the signal derived from the detected ultrasonic signal and two of the reference signals.
In a preferred embodiment, the or each said ultrasonic signal is sequentially transmitted at two carrier frequencies, and said carrier frequencies are selected according to the respective first modulating signal.
Preferably, the or each said first modulation signal is a pseudo-random bit sequence
By providing the or each said first modulation signal in the form of a pseudo-random bit sequence, this has the advantage of enabling a signal suitable for modulation purposes to be produced by relatively simple means and which is suitable for convenient processing by digital circuitry.
The time interval between successive bits of the pseudo-random bit sequence may be a multiple of the periods corresponding to the carrier frequencies, which are preferably substantially 39.6 kHz and substantially 40.4 kHz.
This provides the advantage that in a device of chip width (the time from generating one bit of a digital sequence to the next bit) of 2.5 ms, there are exactly 101 cycles of 40.4 kHz and 99 cycles of 39.6 kHz in one chip. As a result, the two frequencies have the same leading edge where the new chip of the sequence is sent, with the advantage that the transmission of the ultrasonic signal is continuous and has no harmonics.
The method preferably further comprises the step of transmitting a third signal from at least one said second location to a said first location to activate emission of said ultrasonic and radio frequency signals.
This provides the advantage of enabling the transmitters to use a power save mode.
According to another aspect of the present invention, there is provided a transmitter for a distance measurement system, the transmitter comprising a first emitter for emitting an ultrasonic signal, a second emitter for emitting a radio frequency signal, and control means for controlling the delay between emission of said ultrasonic and radio frequency signals.
The transmitter may comprise means for rotating said ultrasonic signal.
Preferably, the transmitter further comprises first modulating means for modulating said ultrasonic signal by a first modulation signal having a repetition period longer than the propagation time of the ultrasonic signal from the first emitter to a detector.
In a preferred embodiment, the first emitter in use emits the ultrasonic signal sequentially at two frequencies selected according to the first modulation signal.
Preferably, the first modulation signal is a pseudo-random bit sequence.
The time interval between successive bits of the pseudo-random bit sequence may be a multiple of the periods corresponding to the two frequencies.
The transmitter may further comprise signal generating means for generating the first modulating signal.
In a preferred embodiment, the signal generating means comprises at least one shift register for receiving the seed of the pseudo-random bit sequence and a gate network connected to an output of the shift register.
Preferably, the gate network comprises a plurality of XOR gates.
In a preferred embodiment, the transmitter further comprises second modulating means for modulating the radio frequency signal with the seed of the pseudo-random bit sequence.
This provides the advantage of enabling a similar pseudo-random bit sequence to be generated at a detector and synchronised with that of the transmitter.
In a preferred embodiment, the transmitter may have an inactive mode in which said ultrasonic and radio frequency signals are not emitted, and may further comprise detector means for detecting a third signal and actuating emission of said ultrasonic and radio frequency signals in response thereto.
According to a further aspect of the present invention, there is provided a detector apparatus for a distance measuring system, the apparatus comprising a first detector for detecting an ultrasonic signal, a second detector for detecting a radio frequency signal, and means for determining a transmitter-detector distance from the time interval between detection of said ultrasonic and radio frequency signals.
In a preferred embodiment, the first detector in use detects a first modulation signal having a repetition period longer than a transmitter-detector time of the ultrasonic signal.
The apparatus preferably comprises frequency discriminator means for detecting the ultrasonic signal transmitted sequentially at two frequencies selected according to the first modulation signal.
Preferably, the first modulation signal is a pseudo-random bit sequence, and the second modulation signal includes the seed of said pseudo-random bit sequence.
The time interval between successive bits of the pseudo-random bit sequence may be a multiple of the periods corresponding to the two frequencies.
In a preferred embodiment, the frequency discriminator means comprises a phase locked loop.
In a preferred embodiment, the second detector in use detects a second modulation signal associated with the first modulation signal.
The determining means may comprise processor means for generating a plurality of reference signals corresponding to the first modulating signal delayed by substantially constant time intervals relative to each other, and comparing the detected first modulating signal with each of said plurality of reference signals.
In a preferred embodiment, the processor, comprises a shift register for receiving the seed of the pseudo-random bit sequence and a gate network for generating the pseudo-random bit sequence therefrom.
Preferably, the gate network comprises a plurality of XOR gates.
The processor may evaluate a degree of correlation between the first modulation signal and each of the plurality of reference signals.
In a preferred embodiment, the processor comprises a micro computer.
According to a further aspect of the present invention, there is provided a distance measuring system comprising at least one transmitter as defined above and at least one detector apparatus as defined above.
According to a further aspect of the present invention, there is provided a distance measuring method comprising the steps of transmitting between at least one pair of first and second locations a respective first signal having a first propagation speed, transmitting between the or each pair of first and second locations a respective second signal having a second propagation speed higher than the first propagation speed, and determining the distance between the first and second locations of the or each said pair of first and second locations from the difference in propagation time between said first and second signals, wherein the or each said first signal includes a respective first modulation signal comprising a pseudo-random bit sequence having a repetition period longer than the propagation time of said first signal between the respective first and second locations and the or each said second signal includes a respective second modulation signal comprising the seed of the pseudo-random bit sequence.
The method may further comprise the step of rotating the or each said first signal. This may be achieved by mechanical means such as a motor or electronic means such as a phased array of transmitters.
This provides the advantage of enabling a position vector between a transmitter and a detector to be determined.
The or each said first and second signal may be emitted from a respective first location and detected at a respective second location. Alternatively, the first and second signals may be emitted from different locations, and suitable means used to exchange information between the locations.
Preferably, the step of detecting the or each said second signal further comprises deriving the respective second modulation signal, generating at the respective second location a plurality of reference signals corresponding to the respective first modulation signals delayed by substantially constant time intervals relative to each other, and comparing a signal derived from the or each said detected first signal with each of the plurality of reference signals.
This provides the advantage that by having a sufficient number of reference signals at the or each second location, detection of substantial correlation between the signals derived from the respective first signals and the reference signals indicates the approximate time delay between receipt of respective first and second signals. This in turn provides the advantage that sophisticated memory devices are not required in order to determine the difference in propagation time between respective first and second signals.
In a preferred embodiment, the step of generating said reference signals comprises generating respective first modulation signals at the or each second location and inputting each said first modulation signal to a respective shift register having a plurality of outputs.
The method may further comprise the step of determining a degree of correlation between the or each signal derived from the detected first signal and each reference signal.
This provides the advantage that when the difference in transmission time between respective first and second signals lies between two of the intervals corresponding to the reference signals, this is readily indicated by correlation between the signal derived from the detected first signal and two of the reference signals.
In a preferred embodiment, the or each said first signal is sequentially transmitted at two carrier frequencies, and said carrier frequencies are selected according to the respective first modulating signal.
The time interval between successive bits of the pseudo-random bit sequence may be a multiple of the periods corresponding to the carrier frequencies, which are preferably substantially 39.6 kHz and substantially 40.4 kHz.
This provides the advantage that in a device of chip width (the time from generating one bit of a digital sequence to the next bit) of 2.5 ms, there are exactly 101 cycles of 40.4 kHz and 99 cycles of 39.6 kHz in one chip. As a result, the two frequencies have the same leading edge where the new chip of the sequence is sent, with the advantage that the transmission of the first signal is continuous and has no harmonics.
The method preferably further comprises the step of transmitting a third signal from at least one said second location to a said first location to activate emission of said first and second signals.
This provides the advantage of enabling the transmitters to use a power save mode.
Preferably, the or each said first signal is an acoustic signal, and preferably an ultra-sonic signal.
In preferred embodiment, the or each said second signal is an electro-magnetic signal, optical or a radio frequency signal.
According to another aspect of the present invention, there is provided a transmitter for a distance measurement system, the transmitter comprising a first emitter for emitting a first signal having a first propagation speed, a second emitter for emitting a second signal having a second propagation speed higher than the first propagation speed, control means for controlling the delay between emission of said first and second signals, first modulating means for modulating said first signal by a first modulation signal comprising a pseudo-random bit sequence having a repetition period longer than the propagation time of the first signal from the first emitter to a detector, and second modulating means for modulating the second signal with the seed of the pseudo-random bit sequence.
The transmitter may comprise means for rotating said first signal.
In a preferred embodiment, the first emitter in use emits the first signal sequentially at two frequencies selected according to the first modulation signal.
The time interval between successive bits of the pseudo-random bit sequence may be a multiple of the periods corresponding to the two frequencies.
The transmitter may further comprise signal generating means for generating the first modulating signal.
In a preferred embodiment, the signal generating means comprises at least one shift register for receiving the seed of the pseudo-random bit sequence and a gate network connected to an output of the shift register.
Preferably, the gate network comprises a plurality of XOR gates.
In a preferred embodiment, the transmitter may have an inactive mode in which said first and second signals are not emitted, and may further comprise detector means for detecting a third signal and actuating emission of said first and second signals in response thereto.
The first emitter preferably comprises means for emitting an acoustic, and preferably an ultra-sonic, signal.
The second emitter preferably comprises means for emitting an electromagnetic, optical or radio frequency signal.
According to a further aspect of the present invention, there is provided a detector apparatus for a distance measuring system, the apparatus comprising a first detector for detecting a first signal having a first propagation speed, a second detector for detecting a second signal having a second propagation speed higher than the first propagation speed, and means for determining a transmitter-detector distance from the time interval between detection of said first and second signals, wherein the first detector in use detects a first modulation signal comprising a pseudo-random bit sequence having a longer repetition period than a transmitter-detector propagation time of the first signal, and the second detector in use detects a second modulation signal comprising the seed of the pseudo-random bit sequence.
The apparatus preferably comprises frequency discriminator means for detecting the first signal transmitted sequentially at two frequencies selected according to the first modulation signal.
The time interval between successive bits of the pseudo-random bit sequence may be a multiple of the periods corresponding to the two frequencies.
In a preferred embodiment, the frequency discriminator means comprises a phase locked loop.
The determining means may comprise processor means for generating a plurality of reference signals corresponding to the first modulating signal delayed by substantially constant time intervals relative to each other, and comparing the detected first modulating signal with each of said plurality of reference signals.
In a preferred embodiment, the processor comprises a shift register for receiving the seed of the pseudo-random bit sequence and a gate network for generating the pseudo-random bit sequence therefrom.
Preferably, the gate network comprises a plurality of XOR gates.
The processor may evaluate a degree of correlation between the first modulation signal and each of the plurality of reference signals.
In a preferred embodiment, the processor comprises a micro computer.
According to a further aspect of the present invention, there is provided a distance measuring system comprising at least one transmitter as defined above and at least one detector apparatus as defined above.